An air-conditioning apparatus often has an outdoor unit disposed outdoors, and there is a case in which refrigerant stagnates in a compressor while the outdoor unit is suspended. For example, in winter when the outdoor air temperature is low, the ambient temperature of the outdoor unit disposed outdoors becomes lower compared with the ambient temperature of the indoor unit disposed indoors. In such a case, a pressure difference may occur between the refrigerant circuit of the indoor unit side and the refrigerant circuit of the outdoor unit side and may result in stagnation of refrigerant on the outdoor unit side with lower pressure. In particular, when refrigerant stagnates in the compressor disposed in the outdoor unit, the refrigerant dissolves into the lubricant oil and concentration of the lubricant oil decreases. This creates a possibility of failure attributed to poor lubrication in the compressor when, at a startup of the air-conditioning apparatus, the lubricant oil flows out of the compressor with the refrigerant.
Hitherto, to attend to the above problem, a method has been adopted in which a compressor is heated to prevent stagnation of refrigerant in the compressor. However, refrigerant does not always stagnate in the compressor while the air-conditioning apparatus is suspended. Thus, viewed from an energy saving perspective, it is preferable that the compressor is heated (preheated) only after a refrigerant stagnating state has been determined by some kind of method. Accordingly, in Patent Literature 1, a method of preheating a compressor is disclosed in which an outdoor unit is provided with an outdoor air temperature sensor and with a temperature sensor of the outer wall of the compressor, each sensor determining whether the inside of the compressor is in a refrigerant stagnating state by using its detection value, and when determined that the compressor is in a refrigerant stagnating state, a motor of the compressor is energized in an open phase state (applying alternating current with a missing phase to the motor so that the motor does not rotate, thus making a coil generate heat), for example.
Further, in Patent Literature 2, a method of preheating a compressor is disclosed in which a compressor is provided with a gas-liquid determination sensor, and when the gas-liquid determination sensor detects that a liquid refrigerant has stagnated more than or equal to a certain liquid surface level in the compressor, a crankcase heater provided in the outer circumference of the compressor is energized.